For more than three decades, the worldwide standard for monitoring fetal condition has been cardiotocography (CTG). Cardiotocography is the simultaneous registration of fetal heart rate and uterine activity. In case of uterine contractions, the umbilical cord may get (partially) occluded. In addition, pressure might be exerted on the fetal head. A healthy fetus generally will respond to these new circumstances by a sudden reduction in fetal heart rate. Whether the fetal heart rate is indeed reduced and also the latency between the actual contraction and the deceleration of fetal heart rate, are important clinical parameters to diagnose fetal wellbeing. Two methods are well-known for monitoring the uterine contractions. The first of these methods consists of an elastic belt that contains a strain gauge to assess the degree of “hardness” of the abdomen. The second method consists of a pressure catheter that is, via the vagina and cervix, inserted into the uterus. The elastic belt with strain gauge is referred to as the tocodynamometer. In case of uterine contractions, the “hardness” of the abdomen increases, which is reflected by the tocodynamometer. With this external method however, it is not always possible to discriminate between uterine contractions and (in)voluntary contractions of the abdominal muscles (e.g. due to movement, coughing, etc.). In addition, in case the tocodynamometer does detect true uterine contractions, both the onset and amplitude of the reflected activity may be unreliable and inaccurate. The invasive catheter, or intrauterine pressure catheter (IUPC), detects the factual pressure inside the uterus, yielding a reliable and accurate representation of uterine activity. However, due to the risks involved, e.g. the catheter has been reported to pierce the placenta, and the fact that the IUPC can only be applied after rupture of the membranes and sufficient dilatation of the cervix, the use of this method in current clinical practice is limited.
Now, from literature it is known that the electrical activity of muscles, recorded as the electromyogram (EMG), can provide an accurate reflection of their mechanical activity. For instance, the contraction and relaxation of the cardiac muscles (myocardium), recorded as a specific variant of the EMG and which is referred to as the electrocardiogram (ECG), is widely used to evaluate the performance of the cardiac muscles. Similarly, the electrohysterogram (EHG; the electrical activity of the uterine muscles or myometrium) is reported to provide information of the performance of the uterine muscles. As a matter of fact, the information in the EHG can be processed and presented in such a way that it mimics the information that can be obtained with the IUPC with the main difference that the EHG can be obtained from standard-issue skin electrodes positioned over the uterus on top of the maternal abdomen. Electrohysterography therefore offers a valuable alternative to the IUPC, as it provides reliable information on uterine activity and is free of risks to mother and fetus and can be applied long before rupture of the membranes.
Several methods and systems have been disclosed that use electrohysterography to provide information on uterine contractions. Most systems are intended to provide a measure for uterine activity, often together with a measure for fetal heart activity, to be used for monitoring fetal condition. The methods that are used to provide this measure for uterine activity are either based on filtering of the electrohysterogram or on spectral analysis of the electrohysterogram. Also, systems have been disclosed that are intended to discriminate between harmless contractions of the uterus and actual contractions that lead to birth. In these systems, either the propagation velocity of the electrical signals across the uterus, or spectral parameters of the electrohysterogram are analyzed.
Published patent application US2012/0150010 discloses an apparatus and a method for detecting uterine activity. The apparatus uses cutaneous electrodes on the maternal abdomen to obtain electrophysiological signals that can be used to obtain fetal and maternal heart rate. The apparatus includes a first input for receiving electrical signals from the cutaneous electrodes and a second input for receiving movement signals indicative of a movement of the maternal body from a movement detector. A signal processor separates a uterine electromyogram signal from fetal and maternal heart rate signals and filters out motion artefacts from the electromyogram using the movement signals. An output presents electrohysterogram (EHG) data from the uterine electromyogram signal.
For monitoring fetal condition, the actual timing between uterine activity and changes in fetal heart rate is of high importance, especially for the diagnosis of late decelerations. Gynaecologists and obstetricians are trained to visually evaluate cardiotocographic recordings that use mechanical measures of uterine activity, either an IUPC or a tocodynamometer. When using electrohysterography for monitoring uterine contractions, it is essential that there is no significant delay in the electrical measure of uterine activity with respect to the mechanical measures, especially at the end of a contraction, as the risk of misinterpretation would exist. However, in all methods that have been proposed, such a delay exists either due to processing that requires a certain amount of data, or due to filter characteristics (such as in the above cited patent application). For example, in the method of the cited patent application, a 0.0166 Hz low-pass filter is used which introduces a relatively large delay in the order of 15 to 20 seconds. As a result, these methods fail to provide an instantaneous output that can directly be used for cardiotocography. Additionally, the electrical measure for uterine activity that is provided by some of the methods suffers from a lack of robustness. Some methods, for example, need to be calibrated for each individual patient or require a learning period for the algorithm that has been used.